I. Field of the Invention
Surgical treatment for cataract(s) represents the most common form of ocular surgery, with almost 3 million procedures performed during 2007 within the United States alone, according to the 2007 Market Scope survey. As people continue to live longer, treatment for cataracts will surely increase in incidence, at least until preventive strategies are discovered. At the same time, major advances in the design and fabrication of intraocular lenses (IOLs) provide the ophthalmologist with an expanding number of options for the patient's presbyopia.
Despite the increased reliance on multifocal and accommodating IOLs, the vast majority of lenses implanted following cataract surgery continue to be monofocal IOLs, and it is estimated that whether accommodative or non-accommodative this trend will remain true for decades to come (Linstrom, 2008). As ophthalmologists have continued to strive to relieve the cataract patient of post operative dependence on spectacle correction, the strategy of mixing and matching intraocular lenses, the timing between surgery as well as which eye to fix first centers around the selection of “ocular dominance.”
New corneal solutions to refractive and presbyopic issues (Presbylasik and Acufocus) as patients preferred or dominant eye. In all of these situations ophthalmologist tends to select the dominant eye for distance and the non dominant eye for near, in part because of the belief that seeing at a distance is more important in patients' everyday lives. Putting aside the validity of this belief in the primacy of distance vision, we are more immediately concerned with the bases for deciding which eye is an individual's dominant eye in the first place.
In the clinic, an individual's dominant eye is conventionally defined using some kind of test that relies on pointing or sighting under conditions where the individual is forced to view an object with one eye (e.g., the Miles A-B-C Method: Miles, 1929). While such tests provide fairly consistent measures of dominance (as evidenced by test/retest reliability), it is arguable whether sighting dominance taps into those sensory/neural processes that are essential for maximizing coordinated binocular vision under conditions where the two eyes are receiving retinal images that differ in spatial frequency content, as they inevitably will when dealing with IOLs. It has long been known (Washburn et al, 1934) that sighting dominance tests can be affected by factors such as handedness that are extraneous for purposes of estimating sensory balance between the two eyes.
II Description of the Known Art
Several recent studies have advocated measures of interocular suppression as valid measures of eye dominance. These recent studies follow the tradition of using relative predominance during binocular rivalry as a means for indexing eye dominance, a tradition that dates back decades (see review by Porac & Coren, 1976). In one study, Valle-Inclan et al. (2008) asked individuals to view two sequences of letters rapidly presented one after the other separately to the two eyes (the authors dubbed this the dichoptic RSVP task, where RSVP stands for rapid serial visual presentation). Following each sequence, people reported whether or not a pre-specified “target” letter was seen on that trial. Some participants only saw a target letter when it was contained within the RSVP stream presented to a given eye, implying that the other eye's view was suppressed under the conditions of dichoptic stimulation; for other people, however, detection performance was equally good regardless of the eye receiving the RSVP stream containing the target, implying that both eyes' views were available to awareness for processing. This technique showed good test/retest reliability, but one has concern whether the RSVP task taps into aspects of interocular suppression plausibly engaged under more sustained viewing conditions. It is known, for example, that streams of transient stimulation can disrupt conventional binocular rivalry (Lee and Blake, 1999), which may explain why a substantial number of participants in the RSVP task described seeing two superimposed letters at the same time. The RSVP task does, however, have the advantage of using an objective performance measure—target detection—as an index of eye dominance.
In another recent study focusing on interocular suppression as an index of eye dominance, Ooi and He (2001) had participants view a briefly presented dichoptic display consisting of an array of six differently colored gratings presented separately to the two eyes; the orientation and color of gratings falling on corresponding retinal areas of the two eyes were dissimilar, creating the stimulus conditions for binocular rivalry. Rather than presenting the rival display for an extended viewing period, Ooi and He presented the array of rival gratings for just 0.33 sec, and following each presentation the participant indicated by pressing one of two keys which they saw, more “red” or more “green”. Over trials the relative intensities of the gratings presented to the two eyes were adjusted to find the so-called balance point where both responses were equally likely. In their sample of several dozen people, balance point values varied from strongly right-eye dominant to strongly left-eye dominant, with some showing essentially perfect balance between the eyes. Interestingly, their measure of eye dominance was unrelated to eye dominance measured using a conventional sighting test, the Ring test (Borish, 1970).
Ooi and He's task is useful in that it permits parametric variation of stimulus strength (intensity, in their study) and stimulus characteristics such as complexity (spatial frequency in their study). However, with their task there is no objectively correct answer on any trial, meaning that each subject must figure out for himself/herself how to judge the strength of a color sensation that will vary over trials unpredictably. This kind of judgment could be confusing to explain and difficult to make for clinical patients, particularly older individuals unfamiliar with vision testing. Moreover, Ooi and He's task uses a very brief exposure duration near the lower limit for producing reliable interocular suppression (Wolfe, 1984; Leonards & Sireteanu, 1993; Blake et al, 2001). Finally, it is likely that their stimulus presentation regime effectively measures biases in initial dominance during rivalry, but it may not tap into neural events responsible for sensory eye dominance operating under more sustained viewing conditions under which the consequences of bilateral IOLs emerge.
In a similar fashion, Handa and colleagues (2004a) quantitatively assessed ocular dominance by manipulating the contrast values of the rival images until they were equally predominant. They were further able to apply this technique to monovision wearers (2004b) and to cataract patients pre- and post-operatively, with the aid of retinometers (2006). Monovision success coincided with smaller differences in contrast thresholds between the monocular images and ocular dominance measures were consistent pre- and post-surgery. In terms of disadvantages in technique, a similar argument can be made here, as with Ooi and He's paradigm. In addition, the size of the stimulus displays were large enough to induce relatively moderate amounts of ‘piecemeal’ rivalry, which increases response uncertainty during binocular rivalry monitoring. Measuring several contrast values for each eye also lengthens testing duration.
A variety of tests have been created to assess ocular dominance (review by Evans, 2007), and more than 25 different types of ocular dominance have also been proposed (Walls, 1951). It is no wonder, therefore, that controversy still exists over which test and type of ocular dominance are most applicable to clinical practice (Evans, 2007; Mapp, Ono & Barbeito, 2003). The types of ocular dominance and corresponding tests have been categorized into three domains: sighting dominance, sensory dominance based on persistence during binocular rivalry, and sensory dominance based on functions inherent to spatial vision, including acuity (Coren & Kaplan, 1973; Suttle et al., 2008). Assessment of dominance within and across these domains has usually lacked agreement (Suttle et al., 2008; Ooi & He, 2001; Walls, 1951; Coren & Kaplan, 1973; Seijas et al., 2007; review by Evans, 2007; Mapp, Ono, & Barbeito, 2003; Pointer, 2007). Similarly in our study, we found little consistency in dominant eye or dominance strength across the hole-in-the-card test, acuity measures, and our interocular suppression task. Overall, this suggests that, for an individual, there is no eye which is clearly superior across all visual functions, and the dominant eye may depend on the test used and function assessed (Suttle et al., 2008; Seijas et al., 2007; Mapp, Ono, & Barbeito, 2003).
One objective here is to produce a technique best suited for determining ocular dominance as a means of successfully implementing monovision with refractive surgery. Monovision correction entails the monocular “fogging” of one eye, usually the non-dominant eye, which is corrected for near vision (Evans, 2007). It is presumed that it is less demanding to suppress a blurred image in the non-dominant than the dominant eye (corrected for distance vision), thus minimizing discomfort for the subject. Indeed, there is evidence to suggest the interocular suppression occurs in monovision (Kirschen, Hung & Nakano, 1999; Simpson, 1991; Schor, Landsman & Erickson, 1987) and ocular dominance may influence one's ability to suppress anisometropic blur in monovision (Evans for review, 2007). In addition, one of the major complaints by monovision patients is the inability to suppress blurred images at night which may also account for the appearance of ghosting or haloes around lights, especially during driving (Evans, 2007). Thus, it is our intuition as well as those of many clinical practitioners that measuring interocular suppression is the most relevant approach in determining ocular dominance in that it best simulates the patients' situation after monovision correction.
Several sensory tests have been created to measure interocular suppression by presenting dissimilar stimuli dichoptically (Ogle, 1962, Collins & Goode, 1994; Ooi & He, 2001; Handa et al., 2004a,b & 2006; Valle-Inclan et al., 2008). For the reasons previously mentioned, these forms of binocular rivalry may be challenging for patients to perform and may not be reliable for a clinical setting (but see Handa et al., 2006). In contrast, our approach is more akin to the technique by Humphriss (1982) whereby individuals interocularly suppressed the lens-induced blurred image without awareness and without ever perceiving rivalry. Interestingly, within the few studies that found agreement among tests of ocular dominance, several found a correlation between sighting eye and dominant eye during rivalry or blur suppression (Spry et al., 2002; Ooi & He, 2001; Handa et al., 2004a; Schor, Landsman & Erickson, 1987; Porac & Coren, 1978). We found a similar consistency but only among individuals with significant interocular differences in suppression. Collins and Goode (1994) found that individuals with matched ocular dominance for sighting and rivalry were better at suppressing blurred information. This may imply that there is a level of suppression involved when individuals' are forced to choose one monocular view over another in a sighting test. This also highlights individual differences in the ability to suppress blur and other scene information and may be predictive of one's suppression abilities after monovision correction.
Promising evidence exists which suggests that patient's success or satisfaction with monovision correction is related to his or her ability to suppress interocular information (Evans for review, 2007). Handa and colleagues (2004b) reported that individuals with weaker sensory dominance, as determined by binocular rivalry, were more likely to be satisfied with intraocular lens monovision. Schor and colleagues (1987) reported that successful long-term monovision individuals were interocularly balanced for blur suppression. But a significant correlation between monovision success and interocular suppression has not always been found (Collins and Bruce, 1994).
Eye Dominance was one visual function to be tested in apparatus disclosed by Task, Henry L.; and Genco Louis V; Jun. 3, 1987, U.S. Statutory Invention Registration Number H293. Illustrated and described therein is a portable boxlike structure having a first and second illuminated visual displays disposed for viewing by respective left and right eyes of a subject. Among the functions to be tested is eye dominance for which the display pair includes two sets of oppositely oriented diagonal lines. There is no disclosure of separate controls for the two displays for left and right eyes.
One objective here is to produce a technique best suited for determining ocular dominance as a means of successfully implementing monovision with refractive surgery. Monovision correction entails the monocular “fogging” of one eye, usually the non-dominant eye, which is corrected for near vision (Evans, 2007). It is presumed that it is less demanding to suppress a blurred image in the non-dominant than the dominant eye (corrected for distance vision), thus minimizing discomfort for the subject. Indeed, there is evidence to suggest the interocular suppression occurs in monovision (Kirschen, Hung & Nakano, 1999; Simpson, 1991; Schor, Landsman & Erickson, 1987) and ocular dominance may influence one's ability to suppress anisometropic blur in monovision (Evans for review, 2007). In addition, one of the major complaints by monovision patients is the inability to suppress blurred images at night which may also account for the appearance of ghosting or haloes around lights, especially during driving (Evans, 2007). Thus, it is our intuition as well as those of many clinical practitioners that measuring interocular suppression is the most relevant approach in determining ocular dominance in that it best simulates the patients' situation after monovision correction.